On terrains which require a higher grip, combine harvesters may be equipped with a pair of track units instead of front wheels. Different types of track units are in use in present day harvesters. Flat track units comprise a set of guide wheels of equal diameter, guiding a rubber track in the shape of an elongated loop, with bogie wheels in between the two guide wheels. One of the two guide wheels acts as the driver wheel via a suitable drive mechanism connected to the harvester's power source. An example of such a flat track unit is shown in U.S. Pat. No. 5,566,773. Triangular track units have two guide wheels resting on the floor and a third guide wheel above these two and centrally placed, the upper guide wheel usually acting as the drive wheel, with the track moving in the form of a triangular loop about these three guide wheels, and equally with bogie wheels in between the two lower guide wheels. The latter type of track unit is illustrated for example in EP-A-2130749. Equivalent trackless ground engaging means exist wherein the two guide wheels that are in contact with the ground are replaced by wheels provided with tyres.
Regardless of the type of track or equivalent unit applied, the track units are usually suspended on the harvester's chassis in a central point, about which a pivoting movement of the entire unit is allowed with respect to the chassis, so that the unit may tilt up and down in order to adapt to the unevenness of the terrain. When the terrain is exceptionally muddy or soft, this mechanism may cause the track unit to become stuck, as it tilts forwards and tends to burrow itself into the terrain.